Mismatch Repair (MMR) is an evolutionary conserved system that targets polymerase mistakes for repair on the newly synthesized DNA strand. Based on bacteria and yeast studies, there appears to be specific recognition patterns by components of DNA MMR, although data on human repair is lacking. In addition to single base pair mismatches that distort the DNA strand, DNA MMR can detect "loops" formed at repetitive sequences termed microsatellites, presumably caused by slippage by the DNA polymerase that leaves at least one extra nucleotide on a DNA strand. The result is a frameshift mutation at the microsatellite, which can be detected electrophoretically. Most microsatellites are in non-coding regions of DNA, but certain tumor suppressor genes contain coding microsatellites that develop frameshift mutations when DNA MMR is impaired. Mutations of PTEN, TGFBR2, ACVR2, and BAX are often found in cancers from patients with Lynch syndrome (caused by germline mutation of a DNA MMR gene), or in sporadic microsatellite unstable colorectal cancers. In this proposal, our overall hypothesis is that the human DNA mismatch repair system has specific recognition fidelity in targeting repair of frameshifted loops. We propose based on the individual defect of the DNA mismatch repair system that non-coding microsatellites will be repaired at varied rates. Similarly, coding microsatellites found in target genes that help determine the enhanced growth of microsatellite unstable cancers will become mutated at varying rates based on the type of mismatch repair defect. We hypothesize that in addition to a specific mismatch repair defect, the surrounding DNA structure may influence the ease or ability of a coding microsatellite to become mutated. The studies proposed here are designed to help understand why and how target genes become mutated in microsatellite unstable cancer. These studies are important to understanding the pathogenesis of microsatellite unstable colorectal cancer, and may provide clues to a mechanism for interrupting the mutation process.